This invention relates to a mobile communication system and, in particular, to transmission power control of a upward communication channel on soft handoff peculiar to a CDMA (code division multiple access) type mobile communication system.
As is well known in the art, various multiple access types have been adopted in a mobile communication system. One of the multiple access types is a CDMA type. The CDMA type mobile communication system assigns to each channel with a particular code, transmits to the same repeater a modulated wave to which a carrier having the same carrier frequency is spectrum-spread with the code, establishes code synchronization in each receiving side, and identifies a desired channel. The CDMA type mobile communication system may be called a SSMA (spread spectrum multiple access) type mobile communication system.
The CDMA type mobile communication system comprises a plurality of terminals (mobile stations) and a plurality of radio base stations each of which serving as a repeater. Each radio base station is called a base transceiver station in the art. As described above, inasmuch as the plurality of terminals carry out communication using the carrier with the same carrier frequency, it is necessary for the CDMA type mobile communication system to uniformly receive energy of the upward communication channel from each terminal communicating with the radio base station without a position of the terminal.
In order uniformly receive energy in the radio base station, the CDMA type mobile communication system carries out transmission power control for the upward communication channel as described in TIA (Telecommunication Industry Association)/EIA (Electronic Industries Association)/IS-95.
It is assumed that a specific terminal carries out soft hand-off with radio base stations covering cells having different cell diameters. In addition, it is assumed that the specific terminal provides access to a particular radio base station covering the cell having a relatively larger cell diameter to depart from an area of soft hand-off. In this event, when an upward transmission power is controlled by a conventional transmission power control method, the specific terminal rapidly raises transmission power therefor. This is because the specific terminal communicates with the particular radio base station alone. In other words, the specific terminal can receive a downward communication channel signal from the particular radio base station alone. Under the circumstances, other radio base stations covering the cells each having a relatively small cell diameter receive larger interference energy from the specific terminal than those from other terminals communicating therewith. In other words, reception ratio Eb/N0 for each other terminal is less than a reference ratio Eb/N0. As a result, each of the other terminals raise their upward transmission power so that the reception ratio Eb/N0 is equal to the reference ratio Eb/N0. This means that interference energy in the cell having the relatively small cell diameter is further raised and it results in decreasing subscriber capacity held in the cell having the relatively small cell diameter. In addition, inasmuch as it is necessary for each other of the other terminals covering the cell having the relatively small cell diameter to raise its upward transmission power, it results in increasing consumed power in each other terminal and in shortening the life of its battery.
Various mobile communication systems related to the present invention are already known. By way of example, Japanese Unexamined Patent Publication of Tokkai No. Hei 5-252,098 or JP-A 5-252,098 discloses a mobile communication system which is capable of covering entirely a town and a premises with large traffic and a suburb and a thinly populated area with small traffic, using the frequency utilizing efficiently, and suppressing increase in the traffic due to hand-off. According to JP-A 5-252,098, the mobile communication system comprises a plurality of cells each of which has at least one radio base station in which mobile stations move in an area covered by the cells. The plurality of cells are made up of cells having various radii from a minimum cell to a maximum cell. In addition, each mobile station comprises means for varying its transmission power in accordance with the size of the cell. Specifically, each mobile station comprises a transmitter section which includes a variable amplifier for increasing and decreasing an amplitude of a signal in accordance with a control signal. However, JP-A 5-252,098 only discloses that each mobile station varies its transmission power in accordance with the size of the cell and is entirely different from a technical idea to vary an upward transmission power of each mobile station in accordance with reception energy in a radio base station from the mobile station (in other words, in accordance with a distance between the radio base station and the mobile station). In addition, JP-A 5-252,098 does not concretely explain how to generate the control signal supplied to the variable amplifier in the transmitter section of the mobile station.
Another example is disclosed in U.S. Pat. No. 4,613,990 issued to Samuel W. Halpern. According to Halpern, power levels in first and second stations in a cellular radiotelephone system are dynamically adjusted from only one of them. The second station receives signals from the first and, on the basis of the level of those signals, first adjusts its own transmitter power level to be within a predetermined range and then directs the adjustment of the power level of the first station to be within a predetermined range. Parameters used in processing call handoffs are also adjusted to correspond to the power level adjustments to prevent false handoffs due to the power level changes. However, Halpern does not perceive the above-mentioned problems in a case where a mobile station departs from an area of soft hand-off and therefore does not provide means for solving the above-mentioned problems.
Japanese Unexamined Patent Publication of Tokkai No. Hei 4-290,098 or JP-A 4-290,098 discloses a mobile communication system which is capable of performing mobile switching without interrupting speech even when the service area of a base station is changed while the speech is being performed in the mobile communication system for broad band cordless telephone in building connected to an exchange and city area. According to JP-A 4-290,098, the above-mentioned purpose can be attained by learning a cell boundary line where electric field strength between base stations are set equal by a central station in advance, and calculating the position and the travel speed of mobile machines by measuring the electric field strength of the mobile machines. In such a way, (a) the arithmetic rule of the mobile machine can be simplified even in the building in which cell shape is complicated, and (b) urgency can be decided by calculating a time when the mobile machine arrives at the boundary line, which reduces arithmetic processing quantity, and also, (c) it is possible to realize soft mobile switching for the switching of the base station with a simple arithmetic rule and to improve speech quality. However, JP-A 4-290,098 only discloses technique to perform mobile switching without interrupting speech even when the mobile machine changes the service area of the base station during telephone conversation and is entirely different from a technical idea to control an upward transmission power of each mobile machine.